The present invention is directed to aromatic polyester resins having improved mechanical and hydrolysis resistant properties.
Aromatic polyester resins find a wide application both in fiber and film production as well as in the injection and blow molding.
The hydrolysis resistance of the prior resins is not, however, sufficiently high. In conditions of excessive hydrolysis, the resins become weak with a remarkable loss of tensile properties (elongation, ultimate tensile stress and strength).
It is known that the addition of liquid crystals (PLC) to the polyester resins does not bring about a significant improvement in the mechanical properties of the fibers and films obtained from them. It is necessary to submit fibers and films to sufficiently high stretching ratios in order to obtain such an orientation of the macromolecules as to cause significant increases in the tensile properties. Sufficiently high stretching ratios are not, however, acceptible because of the poor melt strength of the resins.
It has been found (WO-A-94/26821) that the mechanical properties of fibers and films obtained from aromatic polyester resins added with even small quantities of liquid crystals (less than 5% by weight) can be remarkably improved, even under non-excessive stretching conditions, if the resins undergo a solid state polyaddition (SSPA) in the presence of polyfunctional compounds, such as the dianhydrides of tetracarboxylic acids able to react with the resin end groups. Pyromellitic dianhydride (PMDA) is a representative compound.
The addition of liquid crystals to the polyester resins does not cause considerable improvement in the mechanical properties in the resins after extrusion and in the resins subjected to solid state polycondensation treatments. The presence of the liquid crystals has however, a beneficial effect on the hydrolysis resistance.
The addition of dianhydrides, such as PMDA to resins containing liquid crystals does not improve the hydrolysis resistance and the mechanical properties of the resin after extrusion. The mechanical properties and the hydrolysis resistance are considerably improved if the resin undergoes a solid state polycondensation treatment.